1. Field of the Invention
The present invention relates to a diamond cutting tool, and more particularly, it relates to a cutting tool employing diamond which is formed by vapor deposition for its cutting edge and having excellent wear resistance and chipping resistance, and a method of manufacturing the same.
2. Background Information
Diamond, having high hardness and high thermal conductivity, is particularly useful as a tool material for cutting nonferrous metals such as Al alloys, graphite or ceramics. Diamond materials which are used as tool materials are classified into monocrystal and polycrystalline materials. Monocrystal diamond, which is excellent in physical characteristics, has disadvantages such as an extremely high cost, difficulty in working into a desired shape as a tool material, and cleavage.
On the other hand, polycrystalline diamond materials used as tool materials can be roughly divided into two types. The first one is sintered diamond, which is obtained by sintering fine diamond powder and an iron family metal such as Co under diamond-stable extra-high pressure and high-temperature conditions. Such a sintering technique is described in Japanese Patent Publication No. 52-12126 (1977), for example. It is known that, among commercially available sintered diamond materials, those having particle diameters of not more than several 10 .mu.m have excellent wear resistance with no cleavage phenomenon observed in the aforementioned monocrystal diamond. However, since such sintered diamond contains several to several 10% of binder, a cutting edge of such diamond may be chipped due to diamond particles forming the sintered body falling out during cutting. Such a falling-out phenomenon remarkably appears as the wedge angle of the tool cutting edge is reduced, and it is difficult to ensure a long life of a sharp cutting edge of sintered diamond. Due to the binder contained therein, further, the sintered diamond is so inferior in heat resistance to the monocrystal diamond that the sintered diamond is easily worn during cutting.
On the other hand, vapor-deposited diamond, which is another type of polycrystalline diamond for a cutting tool, is superior in heat resistance and wear resistance to the sintered diamond and is hard to chip, since the same is dense and formed of only diamond. Such vapor-deposited diamond is generally prepared by chemical vapor deposition (CVD) by decomposing and exciting a raw material gas which is mainly composed of hydrocarbon, such as methane and hydrogen, under a low pressure.
For example, a diamond-coated tool comprising a tool substrate which is directly coated with diamond has been developed as a cutting tool employing vapor-deposited diamond. In such a diamond-coated tool, however, adhesion between the tool substrate and the as-formed diamond thin film is important. In a tool prepared by forming a diamond thin film on a substrate of cemented carbide, for example, there has been a problem that the diamond thin film separates from the substrate during cutting.
A diamond cutting tool has also been developed which is prepared by brazing vapor-deposited diamond to a tool substrate. For example, each of Japanese Patent Laying-Open Nos. 1-153228 (1989) and 1-210201 (1989) discloses a technique of brazing a thick film of vapor-deposited diamond as a cutting edge to a tool substrate of cemented carbide for providing a cutting tool. Such a tool suffers no problems of separation of the diamond film, dissimilarly to the diamond-coated tool, and is superior in performance to the conventional tool employing sintered diamond.
A cutting tool having a cutting edge member of vapor-deposited polycrystalline diamond is superior in strength, wear resistance and heat resistance to a conventional tool employing a diamond sintered body. Also in such a cutting tool employing vapor-deposited diamond, a brazing filler metal having a melting point of about 650.degree. C. is employed in order to braze the cutting edge member to a tool substrate, similarly to the tool employing a diamond sintered body. In this case, the cutting tool employing vapor-deposited diamond is easily exposed to thermal damage. Namely, the brazing layer can be damaged by the heat generated from the cutting edge during cutting, very easily as compared with the tool employing sintered diamond, since the vapor-deposited diamond has higher thermal conductivity than the diamond sintered body. When the brazing layer is softened by heat and deformed during cutting, the cutting edge of vapor-deposited diamond is chipped.
Furthermore, when a sharp cutting edge having a wedge angle of not more than 65.degree. is formed in a diamond tool which is manufactured by brazing vapor-deposited diamond to a tool substrate, the cutting edge is easily chipped if high stress or an impact load is applied to the cutting edge during intermittent cutting or cutting of hard ceramics.